Color mixing system

ABSTRACT

A method of proportions of toner particles of different colors to be used in a mixture for toning an electrostatic image, the method comprising: 
     determining a desired color for viewing; 
     determining an illumination under which the desired color is to be viewed; 
     determining a target color for printing which results in the desired color being perceived under the illumination; 
     determining a plurality of sets each consisting of only two available candidate toner particles and optionally black and/or transparent color particles; 
     computing a mixture of toner particles of a set of candidate color to particles only from said plurality of sets that best matches target color, according to a predetermined criteria; and 
     mixing the colored toner particles to form a toner for printing.

RELATED APPLICATIONS

The present application is a US national application of PCT/IL99/00178,filed Mar. 28, 1999.

The present invention is related to the field of printing and inparticular to providing toners with non-standard colors.

BACKGROUND OF THE INVENTION

Most printing systems are based on actual printing of only a limitednumber of different standard colored inks. In general, these inks arechosen such that a wide apparent range of colors may be achieved by socalled “half tone” processes. In half tone printing, the color isachieved by overlaying partial printed areas of the various standardcolors. The percentage of the respective areas printed by the standardcolors is chosen such that, to the eye of viewer, the desired color isachieved. In general, cyan, magenta and yellow, optionally together withblack are the standard colors, with some systems adding additionalcolors, such as one or more of green, orange and purple.

It is known to mix colored inks to achieve similar effects. This issometimes desirable when large areas are to be printed in a given color,since the quality of printing with a single ink is generally better thanwith half-tones. A custom ink may be mixed from the standard inks by (1)determining the proportions of the standard inks required to achieve thecolor; (2) determining the thickness of ink in an area printed with theproportions of the colors; (3) comparing the thickness with a desiredthickness; and (4) mixing the proportions of the standard colors and atransparent ink and/or adjusting the printing to achieve the desiredcolor, preferably with the desired thickness.

Standard systems have been developed by which a wide range of colors canbe achieved by mixing standard colors. Some of these systems are basedon color charts. Others are based on computer programs that define thepercentages of different standard colored inks (including black andtransparent) that should be mixed to achieve a desired color.

It is also known that an approximation of the desired color can beachieved, in a half tone printing process, utilizing only two colors andblack. In some systems five or six colors (plus black) are provided. Adesired color is provided by half toning utilizing only the two neareststandard colors which form a segment in L*H*C* space and black. This twocolor process is described for example in co-pending PCT patentapplication PCT/IL98/00101, filed Mar. 2, 1998 and entitled “ExpandedColor Space”, the disclosure of which is incorporated herein byreference. In general such printing results in colors which match thedesired color only in a particular lighting situation and deviate fromit somewhat in other lighting.

When toners are used for printing, mixing colors is generallyimpractical. In particular, if toner particles of various colors aremixed together and used for printing, the rate of depletion of thedifferent colors will be different, probably at least because themobility of the particles is somewhat different. Thus, with use, thecolor of the toner (and thus of the printed surface) will change as theuneven depletion of the different components of the mix takes place.

SUMMARY OF THE INVENTION

One aspect of some preferred embodiments of the invention is concernedwith mixing toners of different colors to achieve a mixture which stablyprints a target color despite uneven depletion of the components of themixture.

In a preferred embodiment of the invention, according to this aspect,only two colors of toner particles, plus black and transparent are usedto form the mixture. Most preferably, the two colors are colors that arenearby the target color. It has been found that the color achieved whenprinting with such mixtures is more stable than when a greater number ofcolor components or non nearby colors are used. While it is moredesirable to use a greater number of colors in the mixture to achievebetter color fidelity, acceptable fidelity, at least under a givenlighting condition, can be achieved with only two colors.

In a preferred embodiment of the invention, according to this aspect,color matching is made to a color which is the combination of the colorspectra of the actual target color and that of a desired viewingillumination. In a preferred embodiment of the invention, the desiredillumination is chosen from a plurality of different illuminations, byan operator and a spectrum of the target color, as viewed in the desiredillumination, is computed. This specimen defines what will be referredto herein as an “illumination adjusted color.” The two colors are chosenfrom a group of standard colors.

These two colors are preferably those which form a segment of minimumsize in an HC space, in which segment the target illumination adjustedcolor is contained. Such colors are referred to herein as “nearbycolors.” The proportions of the two colors and black and the thicknessof printed required to achieve a best match to the illumination adjustedcolor are computed. To the extent that this thickness is less than thestandard thickness or range of thickness achieved by a printer on whichthe ink will be used, a proportion of transparent ink is computed, suchthat the printed color (with the requisite thickness) will have thedesired hue and saturation.

It should be noted that it is believed that this aspect of the inventionis also applicable to mixing utilizing more than two toner colors.

In preferred embodiments of the invention, additional factors are takeninto account in determining the color mix. These may include, thecoloration of the paper or other substrate on which the color is to beprinted, the difference between the proportions of toner in mixture andon the developed image and the difference between the proportions oftoner left on the developed paper after metering of the developed image.

There is thus provided, in accordance with a preferred embodiment of theinvention a method of determining proportions of toner particles ofdifferent colors to be used in a mixture for toning an electrostaticimage, the method comprising:

determining a target color for matching;

determining at least one set of candidate colored toner particles;

computing a mixture of toner particles of the at least one set ofcandidate color toner particles that best matches target color, and

mixing the colored toner particles to form a toner for printing.

In a preferred embodiment of the invention, determining a target colorcomprises:

determining a target color,

determining an illumination under which the color is to be viewed;

determining the target color for matching as the apparent color of thetarget color under the illumination.

Alternatively, determining a target color comprises:

determining a target color;

determining an illumination under which the color is to be viewed;

determining a color of a substrate on which the target color is to beprinted; and

determining the target color for matching as the apparent color of thetarget color, printed on the substrate, under the illumination.

Preferably, determining the color of a substrate comprises choosing asubstrate type from a plurality of substrate types.

In a preferred embodiment of the invention, determining an illuminationcomprises choosing an illumination from a plurality of illuminationtypes.

In a preferred embodiment of the invention, determining at least one setof candidate color particles comprises choosing two types of particlesboth of which are different from black and transparent. Preferably,determining the at least one set of candidate color particles compriseschoosing said particles from a plurality of available color tonerparticles.

Preferably,determining the at least one set of candidate color particlescomprises choosing the pair of colored toner particles in said pluralityof available color toner particles, which pair comprises colors whichare closest to the target color.

In an alternative preferred embodiment of the invention, determining theat least one set of candidate color particles comprises choosing aplurality of pairs of said particles. In a preferred embodiment of theinvention determining the at least one set of candidate color tonerparticles comprises choosing all pairs of available color particles.

In a preferred embodiment of the invention, the method includes:

determining a best match proportionate mixture for each set of colorpairs which best matches the target color; and

choosing the color pair and proportions having the best match to thetarget color.

Preferably, when more than one color pair have the same best match, thepair having the closest colors is chosen.

In a preferred embodiment of the invention, computing a mixture of tonerparticles comprises comparing the target color to a calculated printedcolor of the color toner mixture under the conditions under which it isto be viewed.

Preferably, the method includes:

correcting the mixture of toner particles to be mixed, responsive to therelative transfer proportions of the toner particles to theelectrostatic image during development of the image.

In a preferred embodiment of the invention the method includes mixingblack and transparent toner particles with the colored toner particlesto form a better color match between the target and a color printed withthe mixture.

In a preferred embodiment of the invention, the toner particles areprovided in separate liquid toner dispersions of toner particles incarrier liquid and mixing the toner particles comprises mixing theliquid toner dispersions.

In a preferred embodiment of the invention the method includescorrecting the mixture of toner particles to be mixed, responsive to theproportion of toner particles of different types removed from adeveloped image during processing of the image.

There is further provided, in accordance with a preferred embodiment ofthe invention, a toner comprising a mixture of toner particles producedaccording to the invention.

There is further provided, in accordance with a preferred embodiment ofthe invention, a method of printing comprising:

providing an electrostatic printing apparatus; and

printing images in the apparatus utilizing toner according to theinvention.

In a preferred embodiment of the invention, the method includesadjusting the thickness of the printed image in order to provide abetter match to the target color.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from the followingdescription of the preferred embodiments thereof, taken together withthe following drawings, in which:

FIG. 1 is a simplified flow diagram of a method of providing custom inkmixtures, in accordance with a preferred embodiment of the invention;and

FIG. 2 shows a simplified schematic drawing of a system for determiningcolor mixing, in accordance with a preferred embodiment of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates a method of determining the amounts of various colorsof ink to be used in mixing a custom ink, in accordance with a preferredembodiment of the invention.

Preferably, the process starts with the selection by a user of a desiredtarget color (10) and a desired illumination (12).

The target color may be chosen in one of a number of ways. One way tochoose the target color is to scan a sample of the color and determineits spectrum. Another way is to choose a color from a color chart.Preferably, a computer, which carries out the calculation anddeterminations described below, will have spectra of these color chartcolors in a memory. A third method would be to define the half-toneprinted proportions for the color or other color component breakdown ofthe color. Yet another way is to utilize the values of three colorparameters (for example L*a*b*) received with the color.

The illumination could be defined by defining a particular standardillumination from a catalog of such illuminations. Such illuminationswould typically include at least some of daylight, fluorescent lighting,incandescent lighting, and other lighting sources as are well known. Thecomputer preferably has spectra of these sources stored in a memory.

Alternatively, the light source may be specified by its spectrum.Alternatively, the light source itself may be provided and measured todetermine its spectrum.

The next step in the process is, preferably, the determination of theillumination adjusted color spectrum (14). This color spectrum is thebasis for the rest of the process. The color coordinates of theillumination adjusted color spectrum are found next (16).

In a preferred embodiment of the invention, the color coordinates arefirst determined in L*a*b* space and then these coordinates are used todetermine the two coordinates H*C* in an L*H*C* space determined by A*B*(18). These are both standard processes, and are well known in the art.In other preferred embodiments of the invention, other color coordinatesystems may be used and other paths to determining the color coordinatesmay be used.

In a preferred embodiment of the invention, in the next step (20),groups of available colors (from a plurality of such available colors)are chosen as candidates for providing the mix. Each said grouppreferably consists of two adjoining colors in the color space. Anattempt is made to match, to the extent possible, the target color to aprinted mixture of colors as printed on the particular substrate and asviewed in the desired illumination. Such computations are well known inthe art and they are, in fact the same as those used in determining theproportions of the areas to be printed, in half-tone printing, toachieve the target color. The L*a*b* of the printed mixture is comparedto that of the adjusted target color spectra and a difference isdetermined. The best match for a particular pair of toner colors ischosen as a candidate mixture (22).

The candidate mixtures are compared and the pair of colors which givesthe best match between the target and printed colors is chosen as thebasis for determining the mixture of the 153 toners (24). Thisautomatically results in the use of the two nearby colors.

Alternatively, candidate mixtures are computed for all of the possiblepairs of colors and not just for the nearby pairs. The color differencesbetween the adjusted target colors and the printed colors (for examplein L*a*b* space) are computed and the mixture with the smallestdifference is chosen. In the event that more than one pair has a minimumvalue, the closeness of the colors in these pairs is determined and theclosest pair of colors is chosen. Preferably, this determination ofcloseness is made taking into consideration illumination and substratecoloration. This situation may arise for example, where more than onepair provides an exact color match.

The present inventor(s) has(have) found that when illumination adjustedspectra are used, the fidelity of the printed color, under the chosenillumination conditions is improved. This is especially true when onlytwo colors plus black are used to form the ink. Furthermore, it has beenfound that for charged toner, and especially for liquid toner, in whichthe different color particles deplete at different rates, the colorremains closer to the desired color when two colors are used than whenthe color is matched more precisely with a greater number of colors. Ithas been found that achieving greater fidelity by using a greater numberof colors results in the fidelity balance being much more sensitive tothe exact mix of colors being maintained.

After the proportions of the colors and black have been determined, thetotal amount of toner or ink particles per unit area required to providethe desired color saturation is determined. In general, the relativesaturation information is stored in the computer as a thickness of inkrequired or as a percent surface coverage required as a function ofsaturation. In general, printers are set to print at a fixed inkthickness. If the total ink thickness required by the components, forthe desired saturation, is less than the desired total ink thickness,then an additional amount of transparent ink is specified as part of themixture (26). It should be noted that the term transparent ink is usedto denote colorless toner, including colorless particles, such as tonerparticles.

If the total required thickness is greater than the thickness deliveredby the printer, then an ink or toner mixture cannot be specified,without either changing the printed thickness or deviating from thedesired target color.

It might be thought that the thicknesses determined in step 26 should bethe proportions of the toner mixture. However, in accordance with apreferred embodiment of the invention, an additional factor (28) istaken into account. This factor is the different mobilities of the tonerparticles (or the relative depletion rates) for the toner particlecomponents. This difference in mobilities causes a compound effect. Thefirst part of this compound effect is that the different mobilitiescause different proportions of toner particles to be deposited in thedevelopment process. These different proportions must be taken intoaccount, by increasing the proportion of less mobile toner particles inthe mixture.

Moreover, these particles are not uniformly distributed in the developedlayer of toner.

In particular, the more mobile toner particles are closer (on theaverage) to the photoreceptor than the less mobile toner particles. Inmany liquid toner systems, the outer, less dense, layers of thedeveloped layer are removed by a process known as metering. This reduceseven further the amount of less mobile toner that must be added to themixture.

It might be thought that this difference between the proportions oftoner in the mixture and the amount of toner in the printed image mightcause an ever deteriorating match between the target and actual colors.However, this problem is mitigated, so long as the metered tonerparticles are discarded, for the following reason. As printingcontinues, the proportion of the more mobile particles in the mixdecreases. However, these particles are more dense near thephotoreceptor, such that their proportion in the image after meteringchanges more slowly than their proportion in the toner. The differencesin the proportions of the toner particles which develops is preferablycorrected when the toner is replenished, by utilizing a replenishmenttoner concentrate having a greater proportion of the mobile tonerparticles.

All that remains, is to mix the colored inks or toners in the specifiedproportions and print (30).

It should be noted that while the invention has been described in thecontext of determining the color components of a custom ink mixture withtwo colors, the method is also generally applicable to both two colorsand a greater number of colors.

FIG. 2 shows a simplified schematic drawing of a system 70, fordetermining color mixing, in accordance with a preferred embodiment ofthe invention.

System 70 includes a computer 32 having a computation unit 34, an inkmemory 36 for spectra or other color information regarding an availableset of colored inks or toners and an illumination memory 38 for holdinginformation regarding the spectra (or other color information) of aplurality of illumination sources. Optionally, computer 32 includes atarget color memory 40 in which color information regarding a number ofcolors is stored. Alternatively or additionally, the computer mayoptionally include one or more of a memory 42 for storage ofprecalculated color characteristics for combinations of illumination andink and/or substrate or toner spectra and/or a memory 44 in whichprecalculated color characteristics for illumination and target colorsis stored.

In addition, in a preferred embodiment of the invention, the computerincludes a series of memories designated by the reference numerals 46-50(although any number of memories may be present) which temporarily holdinformation regarding one or more of the spectrum (or other colorinformation) regarding the target color, the desired illuminationsource, the colors to be mixed to achieve the target color andinformation concerning the illumination adjusted target color and theillumination adjusted colors of the inks or combinations of the inks.

In one preferred embodiment of the invention, a bus or buses 51 is usedto transfer data and commands within the computer and possibly to andfrom peripherals described below. Alternatively an interface is providedfor each of the inputs and outputs to computer 32.

In operation, according to one preferred embodiment of the invention,the desired color is inputted to computer 32 via a user interface 52.User interface 52 preferably includes a display 54, which displayschoices available to the user or mixing or other information for theuser. Preferably, interface 52 also includes a keyboard 56 or otherdevice for inputting user choices or other user information, asindicated below.

In a preferred embodiment of the invention, system 30 also includes acolorimeter 58 which receives a sample of a color, as for example from acolor card or from a printed sheet or photograph and determines thespectrum of the color. The spectrum information is then transferred toone of memories 46-50 and/or to memory 40.

In a preferred embodiment of the invention, the user is asked to choosefrom a saved target color or to input a particular color via calorimeter58. He is also preferably asked to input a desired illumination. Thisdesired illumination is generally specified from one of theilluminations whose characteristics are stored in memory 38, although itis possible to input a particular illumination by illuminating a sheetof white paper with the illumination and determining the spectrum of thereflected light using colorimeter 58.

Similarly, the color or type of substrate to be used is inputted by theuser and spectral information regarding the reflectivity of the paper isstored in a memory. This spectral information may be pre-stored or maybe inputted based on measurements made with calorimeter 58.

Once a target color, a target illumination and a substrate are chosen,arithmetic unit 34 computes the percentages of colored inks or tonersrequired for the custom ink. This calculation is preferably performedutilizing the method described above in conjunction with FIG. 1 or thevariations on that method described above. Alternatively, any methodused to calculate color components from a given color may be used, basedon the illumination adjusted color of the target and or the inks.

In a preferred embodiment of the invention, the display lists theproportions of the inks to be used in the mixture. Alternatively oradditionally, an automatic mixer 60 is used to supply various toner orink colors from sources 62 to a mixing device 64 in which the ink ortoner is mixed. In a preferred embodiment of the invention, the supplyfrom sources 62 and the mixing is controlled by a controller 66 thatreceives mixing information from computer 32.

In an alternative preferred embodiment of the invention, computer 32 isused to supply color coverage information to a RIP device. In onepossible configuration of this device, image color information isreceived from an image memory. For each pixel in the image, the computercalculates a percentage of coverage of the printed surface for each ofthe colors which are to be printed. This computation is performed takinginto account at least one of (and preferably both of) the illuminationadjusted color of the pixel and the illumination adjusted color of theinks used in forming the image. The printing may be constrained to useonly some of the available colors for any particular pixel or that maybe unconstrained, depending on the desired printing characteristics.

The present invention has been described in the context of a number ofpreferred embodiments and variations. It should be understood that theparticular features of these embodiments may be combined in additionalpreferred embodiments of the invention and that some of the features maybe omitted in some preferred embodiments of the invention with orwithout corresponding loss of finction.

The preferred embodiments of the invention are presented forillustrative purposes only and are not meant to limit the scope of theinvention, which is only limited by the claims. The terms “comprise” or“include” or their conjugates, when used herein mean “including but notnecessarily limited to.”

What is claimed is:
 1. A method of determining proportions of tonerparticles of different colors to be used in a mixture for toning anelectrostatic image, the method comprising: determining a desired colorfor viewing; determining an illumination under which the desired coloris to be viewed; determining a target color for printing which resultsin the desired color being perceived under the illumination; determininga plurality of sets each consisting of only two available candidatetoner particles and optionally black and/or transparent color particles;computing a mixture of toner particles of a set of candidate color tonerparticles only from said plurality of sets that best matches targetcolor, according to a predetermined criteria; and mixing the coloredtoner particles to form a toner for printing.
 2. A method according toclaim 1 and including: determining a color of a substrate on which thetarget color is to be printed, wherein determining a target color forprinting comprises determining a target color for printing which resultsin the desired color being perceived when printed on the substrate,under the illumination.
 3. A method according to claim 2 whereindetermining the color of a substrate comprises choosing a substrate typefrom a plurality of substrate types.
 4. A method according to claim 1wherein determining an illumination comprises choosing an illuminationfrom a plurality of illumination types.
 5. A method according to claim 1wherein determining candidate color particles comprises choosing saidparticles from a plurality of available color toner particles.
 6. Amethod according to claim 5 wherein determining the candidate colorparticles of at least some of the sets comprises choosing the pair ofcolored toner particles in said plurality of available color tonerparticles, which pair comprises colors which are closest to the targetcolor.
 7. A method according to claim 5 wherein determining the sets ofcandidate color toner particles comprises choosing all pairs ofavailable color particles.
 8. A method according to claim 7 andincluding: determining a best match proportionate mixture for each setof color pairs which best matches the target color; and choosing thecolor pair and proportions having the best match to the target color. 9.A method according to claim 8 wherein, when more than one color pairhave the same best match, the pair having the closest colors is chosen.10. A method according to claim 1 wherein computing a mixture of tonerparticles comprises comparing the target color to a calculated printedcolor of the color toner mixture under the conditions under which it isto be viewed.
 11. A method according to claim 1 and including:correcting the mixture of toner particles to be mixed, responsive to therelative transfer proportions of the toner particles to theelectrostatic image during development of the image.
 12. A methodaccording to claim 1 wherein the toner particles are provided inseparate liquid toner dispersions of toner particles in carrier liquidand wherein mixing the toner particles comprises mixing the liquid tonerdispersions.
 13. A method according to claim 12 and including correctingthe mixture of toner particles to be mixed, responsive to the proportionof toner particles of different types removed from a developed imageduring processing of the image.
 14. A method of printing comprising:providing an electrostatic printing apparatus; and printing images inthe apparatus utilizing toner prepared according to claim
 1. 15. Amethod according to claim 14 and including adjusting the thickness ofthe printed image in order to provide a better match to the targetcolor.
 16. A method according to claim 7 and including: determining abest match proportionate mixture for each set of color pairs which bestmatches the target color; and choosing the color pair and proportionshaving the best match to the target color.
 17. A method according toclaim 16 wherein, when more than one color pair have the same bestmatch, the pair having the closest colors is chosen.
 18. A methodaccording to any of claims 1-4, 5, 6, 7, 11, or 12-17 including mixingblack and transparent toner particles with the colored toner particlesto form a better color match between the target and a color printed withthe mixture.
 19. A method according to claim 1 wherein an amount of saidtransparent toner is added to provide a printed image having a desiredcolor saturation when printed at a target thickness.